If you've never been taken by a doctored coin, you're lucky. Doctors heal the sick, but coin doctors are hazardous to the health of your wallet.
Doctoring coins has become a growth industry in the past two decades. While there probably are only a handful of actual experts, there are a lot of part-time professionals who do some strange things to coins to enhance their appearance and salability.
I've been authenticating coins for more than three decades, and in that period I've seen some pretty oddball things done to coins in the name of commerce. I would stress 'see,' since much of what I see is going right over the heads of dealers and customers alike. This is because it takes an act of God to get most collectors and dealers to use a magnifier when examining their coins. This is especially true of the older people, who seem to consider resorting to a magnifier as akin to heresy. You can almost hear them saying, "I've looked at coins without a magnifier all my life, and I'm not about to start now." There are even a few whose credo is: "If God had intended us to see things enlarged, he would have made our eyes into magnifiers."
Sadly, it is this very attitude that keeps the coin doctors in business and eating lobster for dinner three or four times a week. They depend on the fact that perhaps one in twenty dealers or collectors will actually take the time to carefully examine a coin with a magnifier. Even more sadly, some of these doctors of coinology are so good that even a magnifier won't catch them.
I had an opportunity to examine several silver (real silver, not clad) dollars several years ago. The dealer who had bought them became suspicious, although he wasn't able to pinpoint evidence of doctoring. I looked at them with a hand lens and then quickly switched to the stereo microscope that I use for most authentication work. Even then I had to twist and turn the coin until I was able to see where some material had been added to the coin to hide some deep scratches and other markings that would have dropped the coin several grades.
The evidence was there, but the next problem was getting a clear photograph of it. With the twin lenses of the stereo microscope the added material showed clearly. However, when taking photos through a microscope you use only half of it, through one lens, so it took some more twisting and turning and adjusting the light source before my photos showed the proof.
If doctoring is that hard to see - and not all of it is that hard - then it's no wonder that the coin doctors are busy. It takes a sharp eye and a suspicious mind to locate doctoring with a microscope in some cases, in others a hand lens will do the job, but how many doctored coins are getting by the unaided 'eyeball?' From all the evidence, a lot.